Hearing: auditory coding
mechanisms
Harmonics/ Fundamentals
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Recall: most tones are complex tones,
consisting of multiple pure tones
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The lowest frequency tone in a sound is
called the fundamental frequency; other
tones are called harmonics or overtones.
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The fundamental frequency is the greatest
common denominator of the other
frequencies.
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Harmonics, fundamentals (cont.)
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Here's a stringed instrument, plucking
out a fundamental tone:
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And here's the same stringed instrument,
showing some of the harmonics:
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Harmonics, fundamentals (cont.)
The fundamental is (generally) the greatest
common denominator of the harmonic
tones.
100, 200, 300 hz?
200, 400, 600 hz?
In the following, the true fundamental is
missing:
200,300,400,500 hz?
250,300,400 hz?
2000,2400hz ?
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Harmonics, fundamentals (cont.)
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The problem of the missing fundamental
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When the fundamental frequency is m issin g
from a complex tone, people report that they
hear it.
ex: men on (older) phones
ex: bad stereo in a noisy car
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So?
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A problem for traditional explanations of
hearing – the region on the basilar membrane
that codes the fundamental is NOT moving
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Harmonics, fundamentals (cont.)
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The problem of the missing fundamental(cont.)
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So if the basilar membrane isn't moving in the
place that represents the fundamental, how
can you still hear it?
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P lace theory vs. frequency theory
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Place theory is the same basilar membrane
theory we talked about before.
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Frequency theory contends that, if sound is a
vibration, then if neurons fire in response,
they should replicate the frequency.
Harmonics, fundamentals (cont.)
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The problem of the missing fundamental(cont.)
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Place theory vs. frequency theory(cont.)
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Here, a particular frequency's signal is
reproduced in the auditory nerve:
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Does it work?
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Harmonics, fundamentals (cont.)
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The problem of the missing fundamental(cont.)
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Place theory vs. frequency theory(cont.)
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Only sort of.
problem: neurons can't fire fast enough to
match tones at higher frequencies than
about 1000 Hz – but we know we can hear
lower frequencies than that (even when they
aren't actually there, if they're a missing
fundamental - freaky!)
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Harmonics, fundamentals (cont.)
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The problem of the missing fundamental(cont.)
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Place theory vs. frequency theory(cont.)
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volley principle: while one neuron is resting,
another is firing. (helps for coding loudness
as well!)
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Harmonics, fundamentals (cont.)
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The problem of the missing fundamental(cont.)
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Place theory vs. frequency theory(cont.)
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So even though we know place theory
works, frequency theory seems to work too
(thanks to the volley principle), and helps
account for TPOTMF
loudness is psychological reaction to amplitude.
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determined by # of neurons firing at a time
(similar to volley principle).
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4 factors influence loudness perception:
1. duration: brief tones are perceived as
quieter.
2.context: tones presented with a background
of noise (static) are perceived as quieter.
loudness is psychological reaction to
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amplitude.(cont.)
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4 factors influence loudness perception(cont.)
3. observer state:
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attention to tones can make them seem
louder
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adaptation: fresh ears perceive sounds as
louder than ears that have been adapted to
noise.
loudness is psychological reaction to amplitude.
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4 factors influence loudness perception(cont.)
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4. frequency: same amplitude sounds louder
at 1000Hz than at 100 Hz. Equal loudness
contours:
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Localization: How do we know where a sound is
coming from?
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most factors are binaural
1.interaural onset difference. How long did it
take the sound to hit your left ear after your
right?
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Binaural localization (cont.)
2.interaural phase difference. compare where
in the cycle a tone is when it hits the ear.
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Binaural localization (cont.)
3.interaural intensity difference. (sound shadow
lowers intensity of some sounds).
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monaural factors
1.movement parallax: nearby sounds change
their location faster than distant sounds.
2.doppler shift: sounds coming towards you
have a higher pitch than sounds moving away
from you.
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binaural factors help left-right location
discrimination- What about up-down?
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Pinna plays a role here: sounds bounce off
folds of pinna, enhancing some frequencies &
decreasing others – called a . . .
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. . .directional transfer function:
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Gardner & Gardner (1973): inserted modelling
compound into peoples pinnae, & measured
ability to localize sounds.
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result: as pinnae were made smoother &
smoother, localization became worse & worse.
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Music, speech
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Musical tones have two
values: height and chroma.
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octave: sounds that live
directly above one another
on the tone helix represent
successive doubling of
frequencies.
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Music, speech (cont.)
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Krumhansl (1983): probe-tone experiment.
Play a chord (to establish a key), then a tone one of the 12 found within the octave of the
key.
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participants rate goodness" of probe tone.
goodness ratings correlate with use of tones
in classical music.
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Music, speech (cont.)
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music can impact emotional states: Evers &
Suhr (2000) showed music can affect
seratonin release in the brain.
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perfect pitch: ability to identify a musical note
even in isolation from others.
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harmonics important for this ability; when
presented with pure tones, accuracy drops to
about 50%.
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Music, speech (cont.)
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nonmusicians have good memory for pitch:
Schellenburg & Trehub (2003) played TV
themes in a newkey: people did better than
expected.
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auditory cortex larger in musicians with perfect
pitch.
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amusia: inability to recognize melodies and
tunes; other auditory perception (speech,
events) unaffected.
List of terms, section 4
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Harmonic
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Interaural onset difference
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Fundamental
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Interaural intensity difference
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Place theory
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Movement parallax
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Frequency theory
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Doppler shift
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Missing fundamental
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Directional transfer function
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Volley principle
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Gardner & Gardner experiment
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Loudness
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Tone height, chroma
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Duration
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Octave
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Context
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Krumhansl experiment
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Observer state
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Evers & Suhr result
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Attention
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Perfect pitch
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Adaptation
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Schellenburg & Trehub result
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Frequency
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Amusia
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Equal loudness contours
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Binaural factors
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Interaural phase difference